1. Technical Field
This invention relates to an apparatus for testing the magnetic characteristics of magnetic layers of a product and, more particularly, to an apparatus that nondestructively tests these characteristics on both sides of a product simultaneously by saturating the product in a uniform magnetic field.
2. Description of the Prior Art
Apparatus for testing the hysteresis loop of magnetic layers is known. Such commercially available apparatus employ inductive techniques and are commercially known as vibrating sample magnetometers or inductive hysteresis loop tracers. Such devices are well known in the art and require a large electromagnetic field in order to test the sample. The sample is placed in the gap of a large electromagnet thus generally requiring destruction of the product from which the sample was taken.
Magneto-optic Kerr effect apparatus is another form of hysteresis loop testing apparatus presently known. One type of this apparatus also requires that the sample be placed in the gap of a large electromagnet thus requiring that the product be destroyed to create the sample. This type of testing apparatus has been reported in IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG. 22, No. 5, Sept., 1986 at pages 662-664 entitled "Magneto-optic Kerr Effect Hysteresis Loop Measurements on Particulate Recording Media" by Dr. Richard M. Josephs, Dr. Charles S. Krafft and Douglas S. Crompton. In this reference the gap length is described as being 7.6 centimeters, which restricts the size of the sample cut from a product. Thus, this apparatus is generally incapable of nondestructively testing large products such as magnetic recording discs in the gap space or outside the deep gap.
Another magneto-optic Kerr effect method of measuring hysteresis curves is described in U.S. Pat. No. 4,816,761. This device uses an electromagnetic ring magnet with a small gap. Although providing a method of developing a magnetic field for the product being tested, the magnetic field created by the ring magnet is highly nonlinear since the flux lines become less uniform as one progresses away from the gap. As a result, the measurement must generally be taken on a precisely located point very close to the gap in the ring magnet. Because of the extreme nonlinearity of the fringing flux, the field at the point of measurement cannot be calibrated with high accuracy.
Advances in magnetic recording technology are leading to higher and higher coercive forces requiring measuring devices to employ higher magnetization fields. It would be extremely desirable to provide a calibratable and repeatable nondestructive hysteresis loop testing apparatus regardless of the principal employed with a uniform high strength magnetic field. It would also be desirable to be able to test the product with an apparatus that provides a treat deal of latitude as to where the point of measurement needs to be located and, optimally, permits simultaneous measurements on both sides of the product to be tested.